The iphone did less than most other smartphones (no 3g or gps or unapproved software). But it was very simple to set up. Put in a sim card and at worst an apn, username and password and your were done.
Ever tried configuring the internet on a windows mobile htc kaiser? With multiple locations and bit its almost impossible even with a walk though. Tried getting a psion to connect to the internet though your phones irda port? Nightmare.
In short there were lots of people with more capable phones prepared to pay for a data plan but almost nobody could get it to work before the iphone.